1. Field of the Invention
The invention relates generally to fuel cells and catalysts used therein
2. Description of the Prior Art
State-of-the-art proton-exchange membrane fuel cells (PEMFCs) contain high loadings of platinum (Pt), making the fuel cells costly and subject to fluctuations in the market availability of the noble metal. By using little or no Pt in fuel cell electrodes, the cost of the fuel cells and imports of noble metals can be drastically reduced.
Researchers recognized years ago that the Pt content of PEMFC electrodes could be reduced by making by dispersing nanoscale Pt particles on a porous, electronically conductive media (Vulcan carbon) and adding a proton conducting media (a perfluorosulfonic ionomer, Nafion®) (Raistrick, U.S. Pat. No. 4,876,115). When surrounded by Vulcan carbon and Nafion, the Pt serves more effectively as an electrocatalyst for hydrogen oxidation and oxygen reduction because there are ample transport paths for protons and electrons. Whereas the catalytic activity of the Pt is critical, the electrode reactions are mediated by the rate of the transport of the gases, protons, electrons, and water to and from the Pt surfaces.
The oxygen reduction reaction (ORR) at a fuel cell cathode is given in Equation (1) and the hydrogen oxidation reaction (HOR) at the anode is in Equation (2).O2+4H++4e−→2H2O  (1)H2→2H++2e−  (2)
For the reactions to proceed unencumbered, the catalyst site must have a supply of oxygen or hydrogen, protons, and electrons, and must be able to transport away water. The reaction above becomes limited when the transport of any of these four species is slow.
Hydrous phosphate and oxides have innate activity and their microporous or open-framework structures enhance proton conduction (see Colomban, Ed., “Proton conductors: solid, membranes and gels—materials and devices,” Cambridge University Press, Cambridge (1992)). Hydrous iron phosphates are known as corrosion barriers, paint additives and friction coatings; anhydrous phosphate-based catalysts can be used for direct conversion of methane into oxygenates and oxidative dehydrogenation (see Otsuka et al., Appl. Catal. A, 222, 145-161 (2001)); and lithiated FePO4 is being tested as a positive electrode in Li-ion batteries (see Padhi et al., J. Electrochem. Soc., 144, 1188-1194 (1997)).